Method of Performing Uplink Synchronization in Random Access Procedure

ABSTRACT

A method and device for performing a contention based random access procedure by a mobile communication terminal in communication with a base station. The method according to an embodiment includes transmitting a random access preamble message to the base station; receiving a random access response from the base station, the random access response including a timing advance command; determining a status of a mobile communication terminal time alignment timer; and ignoring the timing advance command if the mobile communication terminal time alignment timer is determined to be running in the determining step.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is continuation of co-pending U.S. application Ser. No.12/392,654 filed on Feb. 25, 2009, which claims priority to KoreanPatent Application No, 2008-0023807 filed on Mar. 14, 2008. The entirecontents of each of these applications are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to wireless communications, and moreparticularly, to a method of performing uplink synchronization through arandom access procedure in a wireless communication system.

2. Discussion of the Background Art

Third generation partnership project (3GPP) mobile communication systemsbased on a wideband code division multiple access (WCDMA) radio accesstechnology are widely spread all over the world. High-speed downlinkpacket access (HSDPA) that can be defined as a first evolutionary stageof WCDMA provides 3GPP with a radio access technique that is highlycompetitive in the mid-term future. However, since requirements andexpectations of users and service providers are continuously increasedand developments of competing radio access techniques are continuouslyin progress, new technical evolutions in 3GPP are required to securecompetitiveness in the future. Reduction of cost per bit, increase ofservice availability, flexible use of frequency bands, simple structureand open interface, proper power consumption of a user equipment (UE),and the like e defined as requirements.

In general, there are one or more cells within the coverage of a basestation (BS). One cell may include a plurality of UEs. A UE generallyperforms a random access procedure to access a network. The randomaccess procedure is performed for various purposes, for example, foruplink synchronization between the UE and the network, uplink radioresource allocation request, etc.

The random access procedure starts when the UE transmits a random accesspreamble. The UE randomly selects one random access preamble frompredetermined 64 random access preambles, and transmits the selectedrandom access preamble. Although one preamble is randomly selected from64 random access preambles, two or more UE can simultaneously performthe random access procedure by using the same random access preamble.This is called a contention-based random access procedure. On thecontrary, when a dedicated random access preamble is assigned to eachUE, it is called a non-contention based random access procedure.

One of the purposes of performing the random access procedure is toperform uplink synchronization. When collision occurs between UEs in thecontention based random access procedure, whether collision occurs ornot cannot be known until contention resolution is achieved between theBS and the UE. Therefore, even if collision occurs, the UE may performuplink synchronization by using time alignment information transmittedfrom the BS, resulting in incorrect synchronization. The incorrectuplink synchronization performed by the UE may act as interference toanother UE, which may lead to deterioration in reliability of acommunication system.

SUMMARY OF THE INVENTION

The present invention provides a method of preventing a user equipmentfrom performing incorrect uplink synchronization when the user equipmentfails in contention resolution in a random access procedure.

The present invention also provides a method of preventing one userequipment from causing interference to another user equipment whensynchronization correction is not properly achieved in a random accessprocedure.

In an aspect of the invention, there is a method of performing acontention based random access procedure by a mobile communicationterminal in communication with a base station. The method includestransmitting a random access preamble message to the base station;receiving a random access response from the base station, the randomaccess response including a timing advance command; determining a statusof a mobile communication terminal time alignment timer; and ignoringthe timing advance command if the mobile communication terminal timealignment timer is determined to be running in the determining step.

In an aspect of the invention, the timing advance command is configuredto adjust, start or restart the mobile communication terminal timealignment timer.

In an aspect of the invention, the method includes initiating contentionresolution by transmitting a scheduled message to the base station.

In an aspect of the invention, the method includes applying the timingadvance command to the mobile communication terminal time alignmenttimer if the mobile communication terminal time alignment timer isdetermined to be not running or to be expired.

In an aspect of the invention, the step of ignoring the timing advancecommand includes storing the time alignment data received via the randomaccess response.

In an aspect of the invention, the method includes applying the storedtime alignment data if the mobile communication terminal time alignmenttimer expires before contention resolution is completed.

In an aspect of the invention, the method includes discarding the timealignment information and not stopping the mobile communication terminaltime alignment timer if contention resolution fails.

In an aspect of the invention, the step of not stopping the timealignment timer includes maintaining previously assigned resources.

In an aspect of the invention, the step of maintaining previouslyassigned resources includes maintaining a physical uplink controlchannel (PUCCH), sounding reference symbols (SRS), any configureddownlink assignments and uplink grants.

In an aspect of the invention, there is a mobile communication terminalconfigured to perform a contention based random access procedure with abase station. The mobile communication terminal includes a transceiver;a display; and a processor operatively connected to the transceiver anddisplay, the processor including a time alignment timer. The processoris configured to transmit a random access preamble message to the basestation; receive a random access response from the base station, therandom access response including a timing advance command; determine astatus of the time alignment timer; and ignore the timing advancecommand if the mobile communication terminal time alignment timer isdetermined to be running.

In an aspect of the invention, the timing advance command is configuredto adjust, start or restart the mobile communication terminal timealignment timer.

In an aspect of the invention, the processor is configured to initiatecontention resolution by transmitting a scheduled message to the basestation.

In an aspect of the invention, the processor is configured to apply thetiming advance command to the time alignment timer if the time alignmenttinier is determined to be not running or to be expired.

In an aspect of the invention, the processor is configured to store thetime alignment data received via the random access response.

In an aspect of the invention, the processor is configured to apply thestored time alignment data if the time alignment timer expires beforecontention resolution is completed.

in an aspect of the invention, the processor is configured to discardthe time alignment information and not stop the time alignment timer ifcontention resolution fails.

In an aspect of the invention, the processor is configured to maintainpreviously assigned resources if the time alignment timer is notstopped.

In an aspect of the invention, the processor is configured to maintain aphysical uplink. control channel (PUCCH) sounding reference symbols(SRS), any previously configured downlink assignments and uplink grants,if the time alignment timer is not stopped.

In an aspect of the invention, there is a method of performing acontention based random access procedure by a mobile communicationterminal in communication with a base station. The method includestransmitting a random access preamble message to the base station;receiving a random access response from the base station, the randomaccess response including a timing advance command; determining a statusof a mobile communication terminal time alignment timer; and if themobile communication terminal time alignment timer is determined to berunning in the determining step, ignoring the timing advance command,and transmitting a scheduled message to the base station incorrespondence with the running mobile communication terminal timealignment timer.

In an aspect of the invention, if the mobile communication terminal timealignment timer is determined to be not running or to be expired, themethod includes applying the timing advance command to restart themobile communication terminal time alignment timer, and transmitting ascheduled message to the base station in correspondence with therestarted mobile communication terminal time alignment timer.

Even if a user equipment fails in contention resolution in a randomaccess procedure, it is possible to prevent a case where uplinktransmission fails due to incorrect uplink synchronization, which leadsto interference to another equipment.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of a structure of a wireless communicationsystem according to an embodiment of the invention.

FIG. 2 is a diagram showing an example of functional split between anevolved universal terrestrial radio access network (E-UTRAN) and anevolved packet core (EPC) according to an embodiment of the invention.

FIG. 3 is a block diagram showing constitutional elements of an exampleof a user equipment according to an embodiment of the invention.

FIG. 4 is a diagram showing an example of a radio protocol architecturefor a user plane according to an embodiment of the invention.

FIG. 5 is a diagram showing an example of a radio protocol architecturefor a control plane according to an embodiment of the invention.

FIG. 6 shows an example of a structure of a subframe according to anembodiment of the invention.

FIG. 7 is a flow diagram showing a related art random access procedure.

FIG. 8 is a flow diagram showing a random access procedure according toan embodiment of the present invention.

FIG. 9 is a flow diagram showing a random access procedure according toan embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an example of a structure of a wireless communicationsystem according to an embodiment of the invention. The wirelesscommunication system may have a network structure of anevolved-universal mobile telecommunications system (E-UMTS). The E-UMTSmay also be referred to as a long term evolution (LTE) system. Thewireless communication system can be widely deployed to provide avariety of communication services, such as voices, packet data, etc.

Referring to FIG. 1, an evolved-UMTS terrestrial radio access network(E-UTRAN) includes at least one base station (BS) 20 which provides acontrol plane and a user plane.

A user equipment (UE) 10 may be fixed or mobile, and may be referred toas another terminology, such as a mobile station (MS), a user terminal(UT), a subscriber station (SS), a wireless device, etc. The BS 20 isgenerally a fixed station that communicates with the UE 10 and may bereferred to as another terminology, such as an evolved node-B (eNB), abase transceiver system (BTS), an access point, etc. There are one ormore cells within the coverage of the BS 20. Interfaces for transmittinguser traffic or control traffic may be used between the BSs 20.Hereinafter, a downlink is defined as a communication link from the BS20 to the UE 10, and an uplink is defined as a communication link fromthe UE 10 to the BS 20.

The BSs 20 are interconnected by means of an X2 interface. The BSs 20are also connected by means of an S1 interface to an evolved packet core(EPC), more specifically, to a mobility management entity (MME)/servinggateway (S-GW) 30. The S1 interface supports a many-to-many relationbetween the BS 20 and the MME/S-GW 30.

FIG. 2 is a diagram showing an example of a functional split between theE-UTRAN and the EPC according to an embodiment of the invention.

Referring to FIG. 2, slashed boxes depict radio protocol layers andwhite boxes depict functional entities of the control plane.

The BS performs the following functions: (1) functions for radioresource management (RRM) such as radio bearer control, radio admissioncontrol, connection mobility control, and dynamic allocation ofresources to the UE; (2) Internet protocol (IP) header compression andencryption of user data streams; (3) routing of user plane data to theS-GW; (4) scheduling and transmission of paging messages; (5) schedulingand transmission of broadcast information; and (6) measurement andmeasurement reporting configuration for mobility and scheduling.

The MME performs the following functions: (1) distribution of pagingmessages to BSs; (2) security control; (3) idle state mobility control;(4) system architecture evolution (SAE) bearer control; and (5)ciphering and integrity protection of non-access stratum (NAS)

The S-GW performs the following functions: (1) termination of user planepacket for paging; and (2) user plane switching for the support of UEmobility.

FIG. 3 is a block diagram showing constitutional elements of an exampleof a UE according to an embodiment of the invention and configured toperform the methods shown in FIGS. 8-9. A UE 50 includes a processor 51,a memory 52, a transceiver 53, a display 54, and a user interface unit55. Layers of the radio interface protocol are implemented in theprocessor 51. The processor Si provides the control plane and the userplane. The function of each layer can be implemented in the processor51. The memory 52 is coupled to the processor 51 and stores an operatingsystem, applications, and general files. The display 54 displays avariety of information of the LIE 50 and may use a well-known elementsuch as a liquid crystal display (LCD), an organic light emitting diode(OLED), etc. The user interface unit 55 can be configured with acombination of well-known user interfaces such as a keypad, a touchscreen, etc. The transceiver 53 is coupled to the processor 51 andtransmits and/or receives radio signals.

Layers of a radio interface protocol between the UE and the network canbe classified into a first layer (L1), a second layer (L2), and a thirdlayer (L3) based on the lower three layers of the open systeminterconnection (OSI) model that is well-known in the communicationsystem. A physical layer, or simply a PHY layer, belongs to the firstlayer and provides an information transfer service through a physicalchannel. A radio resource control (RRC) layer belongs to the third layerand serves to control radio resources between the UE and the network.The UE and the network exchange RRC messages via the RRC layer.

FIG. 4 is a diagram showing an example of a radio protocol architecturefor the user plane according to an embodiment of the invention. FIG. 5is a diagram showing an example of a radio protocol architecture for thecontrol plane according to an embodiment of the invention. Theyillustrate the architecture of a radio interface protocol between the UEand the E-UTRAN. The user plane is a protocol stack for user datatransmission. The control plane is a protocol stack for control signaltransmission.

Referring to FIGS. 4 and 5, a PHY layer belongs to the first layer andprovides an upper layer with an information transfer service through aphysical channel. The PHY layer is coupled with a medium access control(MAC) layer, i.e., an upper layer of the PHY layer, through a transportchannel. Data is transferred between the MAC layer and the PHY layerthrough the transport channel. Between different PHY layers (i.e., a PHYlayer of a transmitter and a PHY layer of a receiver), data istransferred through the physical channel. In the PHY layer, modulationis pertained using an orthogonal frequency division multiplexing (OFDM)scheme and time and frequency can be utilized as a radio resource.

The MAC layer belongs to the second layer and provides services to aradio link control (RLC) layer, i.e., an upper layer of the MAC layer,through a logical channel. The RLC layer in the second layer supportsreliable data transfer. There are three operating modes in the RLClayer, that is, a transparent mode (TM), an unacknowledged mode (UM),and an acknowledged mode (AM) according to a data transfer method. An AMRLC provides bidirectional data transmission services and supportsretransmission when the transfer of the RLC protocol data unit (PDU)fails.

A packet data convergence protocol (PDCP) belonging to the second layerperforms header compression function. When transmitting an Internetprotocol (IP) packet such as an IPv4 packet or an IPv6 packet, theheader of the IP packet may contain relatively large and unnecessarycontrol information. The PDCP layer reduces the header size of the IPpacket so as to efficiently transmit the IP packet.

A radio resource control (RRC) layer belongs to the third layer and isdefined only in the control plane. The RRC layer serves to control thelogical channel, the transport channel, and the physical channel inassociation with configuration, reconfiguration and release of radiobearers (RBs). An RB is a service provided by the second layer for datatransmission between the UE and the E-UTRAN. When an RRC connection isestablished between an RRC layer of the UE and an RRC layer of thenetwork, it is called that the UE is in an RRC connected mode. When theRRC connection is not established yet, it is called that the UE is in anRRC idle mode.

A non-access stratum (NAS) layer belongs to an upper layer of the RRClayer and serves to perform session management, mobility management, orthe like.

Data is transmitted from the network to the UE through a downlinktransport channel. Examples of the downlink transport channel include abroadcast channel (BCH) for transmitting system information and adownlink-shared channel (DL-SCH) for transmitting user traffic orcontrol messages. User traffic of downlink multicast or broadcastservice or control messages can be transmitted on the DL-SCH or adownlink multicast channel (MCH). Data is transmitted from the UE to thenetwork through an uplink transport channel. Examples of the uplinktransport channel include a random access channel (RACH) fortransmitting an initial control message and an uplink-shared channel(UL-SCH) for transmitting user traffic or control message.

Downlink physical channels are mapped to the downlink transportchannels. Examples of the downlink physical channels include a physicalbroadcast channel (PBCH) mapped to the BCH, a physical multicast channel(PMCH) mapped to the MGR, a physical downlink shared channel (PDSCH)mapped to the PCH and the DL-SCH, and a physical. downlink controlchannel (PDCCH) for transmitting control information (e.g., downlink(DL)/uplink (UL) scheduling grant, etc.) which is provided from thefirst layer and the second layer. The PDCCH is also referred to as adownlink L1/L2 control channel. Uplink physical channels are mapped tothe uplink transport channels. Examples of the uplink physical channelsinclude a physical uplink shared channel (PUSCH) mapped to the UL-SCH, aphysical random access channel (PRACH) mapped to the RACH, and aphysical uplink control channel (PUCCH) for transmitting controlinformation (e.g., hybrid automatic repeat request (HARQ) acknowledgment(ACK)/negative-ACK (NACK) signals, a scheduling request signal, achannel quality indicator (CQI), etc.) which is provided from the firstlayer and the second layer.

FIG. 6 shows an example of a structure of a subframe according to anembodiment of the invention.

Referring to FIG. 6, a subframe includes a plurality of OFDM symbols anda plurality of subcarriers. The subframe is a basic unit of radioresource allocation. One subframe consists of a plurality of resourceblocks. One resource block includes a plurality of subcarriers (e.g., 12subcarriers). The subframe can be divided into a region where a physicaldownlink control channel (PDCCH) (also referred to as an L1/L2 controlchannel) transmitted and a region where a physical downlink sharedchannel (PDSCH) is transmitted. For example, the PDCCH may correspond tofirst three OFDM symbols in the subframe. A time for transmitting onesubframe is referred to as a transmission time interval (TTI). Forexample, 1 TTI may be 1 millisecond (ms). One subframe can be dividedinto two slots in the time domain. If 1 TTI=1 ms, one slot has a lengthof 0.5 ms.

FIG. 7 is a flow diagram showing a related art random access procedure.

The random access procedure may be performed for the following purposes:(1) an initial access process; (2) a handover process; (3) a process oftransmitting downlink data to a UE that is not time synchronized; (4) aprocess of transmitting data in uplink by the UE that is not timesynchronized; and (5) a recovery process performed when an error occursin a wireless connection.

Referring to FIG. 7, a UE randomly selects a random access preamble froma plurality of available random access preambles. The information togenerate the plurality of available random access preambles can bereceived as a part of system information. The UE transmits the randomlyselected random access preamble to the BS by using a PRACH resource(step 710).

Upon receiving the random access preamble from the UE, the BS transmitsa random access response to the UE through a DL-SCH (step 720). Therandom access response may include time alignment information (i.e., atime advance value or timing advance) for correcting uplink timealignment, uplink radio resource allocation, an index of the randomaccess preamble (i.e., preamble identifier (Id)), a temporary cell-radionetwork temporary identifier (C-RNTI), etc. The random access responseon the DL-SCH can be indicated by a PDCCH addressed by a random accessidentifier. The random access identifier is also referred to as a randomaccess-radio network temporary identifier (RA-RNTI).

After receiving random access response, the UE can correct timealignment based on. the time alignment information. However, in anembodiment of the present invention, instead of correcting timealignment immediately after the UE receives the random access response,time alignment information may be temporarily stored so that timealignment can be corrected by using the time alignment information whena certain condition (i.e., contention resolution, etc.) is satisfied.This process will be described below in greater detail with reference toFIG. 8.

The UE transmits a scheduled message to the BS through a UL-SCH by usingthe uplink radio resource allocation in the random access response (step730).

Upon receiving the scheduled message, the BS transmits a contentionresolution message to the UE (step 740).

Now, contention resolution performed in a random access procedure willbe described in detail.

Collision occurs in the random access process because the number ofavailable random access preambles is limited while a large number of UEsare used. That is, it is difficult for the BS to provide respectivededicated random access preambles to all UEs. Therefore, one UE needs torandomly select and transmit one of random access preambles providedalso to other UEs. Accordingly, in some cases, two or more UEs mayselect and transmit the same random access preamble by using the samePRACH resource. This is called a contention state.

The BS regards two or more identical random access preambles transmittedfrom two or more UEs as one random access preamble transmitted from oneUE. Further, the BS transmits to the UEs the same random access responsefor the received random access preamble.

When collision occurs, the same random access response is received bythe two or more UEs, and the respective UEs perform different operationsaccording to one random access response. That is, by using the uplinkradio resource allocation in the random access response, the UEstransmit different scheduled messages. Accordingly, the UEs may fail totransmit the scheduled message, or only a specific UE may succeed intransmitting the scheduled message according to a location or a transmitpower of each UE.

In the latter case where only the specific UE succeeds in transmittingthe scheduled message, two or more UEs determine that they succeed indata transmission. Therefore, the BS has to inform the UEs, which havefailed in contention, of the fact that data transmission has failed.That is, contention resolution is an operation of informing a UE ofwhether contention caused by collision is successful or unsuccessfulwhen collision occurs between UEs in the random access procedure.

There are two conventional contention resolution methods, that is, afirst method of using a contention resolution tinier and a second methodof transmitting an identifier of a UE which succeeds in contention.

In the first conventional method, it is required that the UE already hasa unique cell identifier (e.g., C-RNTI) before a random access procedureis performed. The UE which already has its cell identifier transmitsdata including the cell identifier to the BS according to a randomaccess response message, and then starts the contention resolutiontimer. If the UE receives the data including the cell identifier througha PDCCH before the contention resolution timer expires, the UEdetermines that the UE succeeds in contention and thus successfullyfinishes the random access procedure.

On the contrary, in the first method, if the UE fails to receive thedata including the cell identifier through the PDCCH before thecontention resolution timer expires, the UE determines that the UE failsin contention. Then, the UE may re-perform the random access procedureor may provide a failure report to an upper layer.

The second conventional method of the two contention resolution methodsis used when the UE does not have its unique cell identifier before therandom access procedure is performed. That is, if the UE does not haveits cell identifier, the UE transmits data including an S-temporarymobile subscriber identity (S-TMSI) or a random Id which is ahigher-level identifier than a cell, identifier according to uplinkradio resource allocation in the random access response, and then startsthe contention resolution timer.

If the data including the higher-level identifier is transmitted througha DL-SCH before the contention resolution timer expires, the UEdetermines that the random access procedure is successful. On thecontrary, if the UE fails to receive the data including the higher-levelidentifier through the DL-SCH before the contention resolution timerexpires, the UE determines that the random access procedure isunsuccessful.

Now, time alignment and time alignment correction will be described. Therandom access procedure is one of methods for uplink time alignment.

When collision does not occur, a time alignment correction method isperformed according to the random access procedure as follows. A BS canmeasure a time alignment value by using a random access preambletransmitted from a UE, and provide time alignment information for timealignment correction to the UE. Upon receiving the random accessresponse, the UE applies the received time alignment information, andstarts or restarts a time alignment timer.

It is assumed that time alignment is maintained between the UE and theBS while the time alignment timer is running. Further, it is alsoassumed that time alignment is not maintained between the UE and the BSwhen the timer alignment timer expires. The BS can measure a timealignment value of the UE by using another method other than the methodof using the random access preamble. The BS can optionally provide thetime alignment information to the UE.

Upon receiving the time alignment information, the UE applies thereceived time alignment information, and starts or restarts the timealignment timer. When the time alignment timer expires, it is prohibitedfor the UE to perform uplink transmission except for transmission of therandom access preamble.

In the random access procedure, there already exists a possibility ofcollision. As a result, incorrect time alignment information may beapplied to the UE due to collision. That is, if time alignment isachieved before the UE transmits the random access preamble, incorrectuplink transmission may be caused due to collision. Accordingly, timealignment is corrected using incorrect time alignment information.

In the following description, it is assumed that uplink time alignmentis achieved between the UE and the BS. In this case, time alignmenttimers used in the UE and the BS are synchronized with each other. It isalso assumed that uplink data transmission is requested in a state wherethe UE does not have an uplink radio resource, and thus the UE performsa random access procedure.

The UE transmits a random access preamble to the BS, and receives arandom access response. The UE applies time alignment informationincluded in the received random access response to the UE itself, andstarts the time alignment timer.

If collision occurs and thus the received random access response is arandom access response to be transmitted to another UE, the UE mayreceive incorrect time alignment information. However, since the timealignment information included in the random access response is appliedto the HE and thus the time alignment timer is running in the UE, the UEdetermines that time alignment is achieved between the UE and the BS.Therefore, if data reception is requested after the UE receives randomaccess response information, the UE attempts uplink transmissionaccording to the incorrect time alignment, resulting in interference totransmission of other UEs.

Accordingly, when the random access procedure is performed by the UEwhich is time-aligned with the BS, the time alignment informationincluded in the random access response is applied after contentionresolution is completed. That is, the UE applies the time alignmentinformation after collision occurs, and then can start or restart thetime alignment timer.

FIG. 8 is a flow diagram showing a random access procedure according toan embodiment of the present invention. This is a case where a UEperforming the random access procedure succeeds in contention caused bycollision.

Referring to FIG. 8, the UE operates in a state where uplink timealignment is achieved between the UE and a BS and a time alignment timeris running (step 810). That is, the UE has a valid time alignment value,and the time alignment timer is running in the UE.

When uplink data transmission is requested in a situation where there isno uplink radio resource, the UE starts a contention based random accessprocedure. Accordingly, the UE transmits a selected random accesspreamble to the BS (step 820). The BS receives the random accesspreamble from the UE and then transmits a random access response to theUE in response thereto (step 830). The random access response mayinclude radio resource allocation for transmitting a scheduled message,a preamble identifier, time alignment information (e.g., a timingadvance command), a temporary C-RNTI, etc.

If, upon receipt of the time alignment information, the UE's timealignment timer is determined to be running, the UE neither applies thereceived time alignment information nor starts the time alignment timer(e.g., until contention resolution is completed). Instead received timealignment information may be stored in a buffer and the UE maintains aprevious time alignment value used before the random access procedure isperformed and also maintains an operation of the time alignment timerwhich is currently running. In other words, the UE ignores the receivedtime alignment information and maintains a previous time alignment valueused before the random access procedure is performed and also maintainsan operation of the time alignment timer that is currently running. TheUE may apply the stored time alignment data if the mobile communicationterminal time alignment timer expires before contention resolution iscompleted.

The UE transmits to the BS the scheduled message including an identifier(e.g., C-RNTI, S-TMSI, random Id, etc.) by using the radio resourceallocation information (i.e., UL grant information) included in thereceived random access response (step 840). Thereafter, the UE receivesa contention resolution message before a contention resolution timerincluded in the UE expires (step 850).

If the UE receives a decodable contention resolution message before thecontention resolution timer expires, the UE determines that the UEsucceeds in contention caused by collision. That is, if the receivedcontention resolution message is a PDCCH including a cell identifier ofthe UE or a DL-SCH including a higher-level identifier (S-TMSI or randomId) of the UE, the UE determines the UE succeeds in contention.

If, after receipt of the time alignment information, the UE's timealignment timer is determined to be not running or to be expired, the UEapplies time alignment information obtained from random accessinformation to a previous time alignment value. Then, the UE restartsthe time alignment timer (step 860).

FIG. 9 is a flow diagram showing a random access procedure according toan embodiment of the present invention. This is a case where a UEperforming the random access procedure fails in contention caused bycollision.

Referring to FIG. 9, the UE operates in a state where uplink timealignment is achieved between the UE and a BS and a time alignment timeris running (step 910). The UE transmits a selected random accesspreamble to the BS (step 920). The BS receives the random accesspreamble from the UE and then transmits a random access response to theUE in response thereto (step 930). The UE neither applies the receivedtime alignment information nor starts the time alignment timer untilcontention resolution is completed. The received time alignmentinformation is stored in a buffer. The UE maintains a previous timealignment value used before the random access procedure is performed andalso maintains an operation of the time alignment timer which iscurrently running. The UE transmits to the BS a scheduled messageincluding an identifier by using the radio resource allocationinformation (i.e., UL grant information) included in the received randomaccess response (step 940).

After receiving the scheduled message, the UE starts a contentionresolution timer (step 950). Unlike the embodiment of FIG. 8, the UEcannot receive its contention resolution message from the BS after thecontention resolution timer starts until the contention resolution timerexpires. For example, a cell identifier or a higher-level identifier ofthe UE may not be included in the contention message, or the contentionmessage including the cell identifier or the higher-level identifier ofthe UE may not be received by the UE before the contention resolutiontimer expires. In this case, the UE determines that the UE fails incontention.

When the contention resolution timer expires, the UE does not apply thereceived time alignment information but discards the time alignmentinformation (step 960) but does not stop the time alignment timer.Accordingly, a previous operation of the time alignment timer ismaintained without alteration. Thus, previously assigned resources(e.g., PUCCH, sounding reference symbols (SRS), any previouslyconfigured downlink assignments and uplink grants) are maintained.

Because the received time alignment information is not applied when theUE fails in contention, transmission error or interference can bereduced when uplink synchronization is incorrectly achieved.

The methods of FIGS. 8-9 may be performed in a device similar the onedepicted in FIG. 3, or in another device. The present invention can beimplemented with hardware, software, or combination thereof. In hardwareimplementation, the present invention can be implemented with one of anapplication specific integated circuit (ASIC), a digital signalprocessor (DSP), a programmable logic device (PLD), a field programmablegate array (FPGA), a processor, a controller, a microprocessor, otherelectronic units, and combination thereof, which are designed to performthe aforementioned functions. In software implementation, the presentinvention can be implemented with a module for performing theaforementioned functions. Software is storable in a memory unit andexecuted by the processor. Various means widely known to those skilledin the art can be used as the memory unit or the processor.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of theinvention as defined by the appended claims. The exemplary embodimentsshould be considered in descriptive sense only and not for purposes oflimitation. Therefore, the scope of the invention is defined not by thedetailed description of the invention but by the appended claims, andall differences within the scope will be construed as being included inthe present invention.

What is claimed is:
 1. A method of performing a random access procedureby a mobile terminal in communication with a base station, comprising:starting a time alignment timer upon applying first time alignmentinformation that is received from a base station, the time alignmenttimer being used to control how long the mobile terminal is considereduplink time aligned; transmitting a random access preamble to the basestation, wherein the transmitted random access preamble is selected bythe mobile terminal; receiving from the base station a random accessresponse as a response to the random access preamble, the random accessresponse including an uplink resource assignment and second timealignment information; wherein the random access procedure is acontention based random access procedure, and: if the time alignmenttimer is running and contention resolution has not been completed,ignoring the second time alignment information even if the mobileterminal is not time aligned with the base station; and if the timealignment timer is not running, applying the second time alignmentinformation.
 2. The method of claim 1, further comprising: if the uplinkis synchronized, ignoring the time alignment information.
 3. The methodof claim 1, further comprising: initiating contention resolution withanother mobile terminal trying the random access procedure bytransmitting a scheduled message using the uplink resource assignment tothe base station.
 4. The method of claim 1, wherein the random accesspreamble is randomly selected by the mobile terminal from a plurality ofavailable random access preambles when the random access procedure iscontention based.
 5. The method of claim 1, further comprising:receiving a message from the base station before transmitting the randomaccess preamble, the message comprising second time alignmentinformation; and applying the second time alignment information foruplink synchronization.
 6. The method of claim 5, wherein the message isnot a random access response.
 7. The method of claim 1, furthercomprising: applying the time alignment information when the randomaccess procedure is noncontention based.
 8. A mobile terminal configuredto perform a random access procedure with a base station, the mobileterminal comprising: a radio frequency (RF) unit; and a processoroperatively connected to the display and configured to: start a timealignment timer upon applying first time alignment information that isreceived from a base station, the time alignment timer being used tocontrol how long the mobile terminal is considered uplink time aligned,transmit a random access preamble to the base station, wherein thetransmitted random access preamble is selected by the mobile terminal,receive from the base station a random access response as a response tothe random access preamble, the random access response including anuplink resource assignment and second time alignment information,wherein the random access procedure is a contention based random accessprocedure, and: if the time alignment timer is running and contentionresolution has not been completed, ignoring the second time alignmentinformation even if the mobile terminal is not time aligned with thebase station, and if the time alignment timer is not running, apply thesecond time alignment information.
 9. The mobile terminal of claim 8,wherein the processor is configured for ignoring the time alignmentinformation if the uplink is synchronized.
 10. The mobile terminal ofclaim 8, wherein the processor is configured to initiate contentionresolution with another mobile terminal trying the random accessprocedure by transmitting a scheduled message using the uplink resourceassignment to the base station when the uplink is synchronized.
 11. Themobile terminal of claim 8, wherein the processor is configured torandomly select the random access preamble from a plurality of availablerandom access preambles when the random access procedure is contentionbased.
 12. The mobile terminal of claim 8, wherein the processor isconfigured for: receiving a message from the base station beforetransmitting the random access preamble, the message comprising secondtime alignment information; and applying the second time alignmentinformation for uplink synchronization.
 13. The mobile terminal of claim8, wherein the processor is configured for applying the time alignmentinformation when the random access procedure is non-contention based.14. The method of claim 1, further comprising: if the time alignmenttimer is running, storing the second time alignment information andsubsequently applying the stored time alignment data after the timealignment timer expires.
 15. The mobile terminal of claim 8, wherein, ifthe time alignment timer is running, the processor is further configuredto store the second time alignment information and subsequently applythe stored time alignment data after the time alignment timer expires.